Class-D amplifiers are classified into a separately-excited oscillation type and a self-oscillation type. Self-oscillating class-D amplifiers are advantageous over separately-excited oscillation class-D amplifiers in terms of audio performance because a large feedback amount can be set in the audible frequency range. Having a self-oscillation loop, self-oscillating class-D amplifiers oscillate in a self-excited manner. Examples of the method of causing self-oscillation using a self-oscillation loop are a method in which self-oscillation is attained by causing positive feedback by rotating the phase of a feedback signal of a self-oscillation loop and a method in which self-oscillation is attained using an integration circuit which performs integration on an analog input signal and a feedback signal of a self-oscillation loop and a hysteresis comparator which receives an output signal of the integration circuit and outputs a binary signal. Among conventional techniques employing the former self-oscillation method are ones disclosed in the following Patent documents 1-3. And among conventional techniques employing the latter self-oscillation method are one disclosed in the following Patent document 4.
In self-oscillating class-D amplifiers, the oscillation frequency varies depending on the use environment (e.g., temperature variation and supply voltage variation) and due to variations with age etc. Therefore, a beat sound is prone to occur when plural self-oscillating class-D amplifiers are installed close to each other and operate simultaneously. In view of this, Patent documents 3 and 4 propose techniques for stabilizing the oscillation frequency of a self-oscillating class-D amplifier. More specifically, in the technique disclosed in Patent document 3, a signal of an external oscillator is input to a halfway point of the circuit path of a self-oscillating class-D amplifier and the oscillation frequency of the self-oscillating class-D amplifier is equalized forcibly with the frequency of the external oscillator (injection locking), whereby the oscillation frequency is stabilized. In the technique disclosed in Patent document 4, a reference voltage of a hysteresis comparator provided in a self-oscillating class-D amplifier is variably controlled so as to become equal to a voltage that is calculated using an input signal voltage value and a drive voltage value of a power amplifier, whereby the oscillation frequency is stabilized.